This invention relates to methods and apparatus for applying cardioversion and defibrillation shocks to a patient's heart, and more particularly, to methods and apparatus for applying charge-balanced shocks (waveforms) to the heart.
Cardiac stimulating devices such as pacemakers and cardioverter-defibrillators are well known. Typically, cardiac stimulating devices contain sensing circuitry for monitoring the various heartbeat signals produced by a patient's heart. Cardiac stimulating devices with sensing circuitry can analyze the patient's heartbeat signals to determine when and at what energy level any electrical pulses should be applied to the heart.
Some cardiac stimulating devices can determine whether the patient is suffering from an arrhythmia such an episode of tachycardia or a fibrillation event. When an arrhythmia is detected, a cardiac stimulating devices may attempt to terminate the arrhythmia by applying electrical pulses to the patient's heart. These pulses may be in the form of high energy cardioversion or defibrillation shocks. Cardioversion pulses have energies in the range of about 2-5 J. Typical defibrillation shocks have energies in the range of about 30-40 J.
Shocks are applied to the patient's heart via leads and electrodes. It is well known that when current is passed through an electrode submerged in an electrolyte such as interstitial fluid or blood, the electrode/electrolyte interface polarizes. The shock-induced polarization produces a voltage rise that can obscure the patient's heartbeat signals. This effect, which is commonly known as "post shock block," is most dramatic in systems in which the electrodes used to apply the shock are adjacent to, or the same as, the electrodes used to sense the heartbeat signals.
As a result of post shock block, whenever a cardioversion or defibrillation shock is applied to the heart it becomes impossible for the sensing circuitry to detect the heartbeat signals until the blocking voltage decays. During the 5-30 second period before the shock induced voltage decays, the cardiac stimulating device is not able to monitor the patient's condition. The cardiac stimulating device is therefore unable to determine whether or not the shock that was just applied was successful at terminating the arrhythmia. A failure to successfully terminate a tachycardia or a fibrillation might require that a more aggressive therapy be applied in a further attempt to terminate the arrhythmia. Unfortunately, due to post shock block, it is not possible to determine the appropriate course of action until the polarization voltage induced by the shock decays.
What is therefore needed is a way in which to reduce the effects of post shock block.